8-Hydroxyquinoline (also known as oxine) is an organic compound derived from the heterocycle quinoline. A colorless solid, its conjugate base is a chelating agent, which is used for the quantitative determination of metal ions.
In aqueous solution 8-hydroxyquinoline has a pK<sub>a</sub> value of ca. 9.9 It reacts with metal ions, losing the proton and forming 8-hydroxyquinolinato-chelate complexes.
The aluminium complex, is a common component of organic light-emitting diodes (OLEDs). Substituents on the quinoline ring affect the luminescence properties.
In its photo-induced excited-state, 8-hydroxyquinoline converts to zwitterionic isomers, in which the hydrogen atom is transferred from oxygen to nitrogen.
8-hydroxyquinoline was first obtained by Hugo Weidel and his student Albert Cobenzl in 1880. They decarboxylated so-called oxycinchoninic acid (from cinchonine) and characterized the resulting compound as melting at about 70ðC. They identified that the hydroxy group is on the benzene ring (but not its particular place) and called the compound oxyquinoline and ñ-quinophenol.
In the following year more chemists found other ways to make the compound. Zdenko Hans Skraup discovered a way to synthesize substituted quinolines from substituted phenols and described three isomers of oxyquinoline, identifying the structure of 8-hydroxyquinoline. and his student Karl Bedall made the compound from a sulphonic acid independently at about the same time, but misidentified its structure.
By 1888 azo dyes were made from the compound.
In the 1920s insoluble chelates of 8-hydroxyquinoline were discovered.
The complexes as well as the heterocycle itself exhibit antiseptic, disinfectant, and pesticide properties, functioning as a transcription inhibitor. Its solution in alcohol is used in liquid bandages. It once was of interest as an anti-cancer drug.
A thiol analogue, 8-mercaptoquinoline is also known.
The roots of the invasive plant Centaurea diffusa release 8-hydroxyquinoline, which has a negative effect on plants that have not co-evolved with it.